The concept of profile-controlling the position of a torch so that it stays on a predetermined weld line, by detecting the arc current flowing through the welding torch, is alread known in the art.
According to one specific method, the detected value of the arc current is compared with a predetermined constant reference value, and the position of the welding torch is controlled in accordance with the result of the comparison. If the welding torch is weaving, for example, the detected value of the arc current at the center of the weaving pattern is compared with a predetermined reference value corresponding thereto, and the position of the welding torch is controlled in accordance with the result of the comparison.
Another specific method notes the weaving of the welding torch, in which the detected values of the arc current at either end of the weaving pattern are compared with each other by using one as a reference quantity, and control is effected in accordance with the result of the comparison. This method makes use of the phenomenon that, if the path of the welding torch strays to the right (or left) off the weld line along the groove, the arc current at the right end (or at the left end, respectively) of the weaving pattern becomes larger than that at the other end. However, this control system has a defect in that the arc is liable to break down. This is especially prominent under conditions in which the arc has a low stability, when the welding current is less than 200 .ANG. or more than 300 .ANG., for example.
Moreover, under welding conditions producing a low arc stability, an unstable breakdown of arc takes place. If this phenomenon occurs, a prior-art control system will attempt to correct the relative position of the welding torch and the weld line, even though the welding torch has not deviated from the weld line, in accordance with the change in the electrical quantity due to the breakdown. Therefore, especially when conditions producing such frequent arc breakdowns continue for a long time, the welding torch often deviates from the weld line and, worse, can completely separate from the weld line so that it is difficult for the welding torch to follow the weld line.